This invention relates to a pick-up device for reading out information which has been recorded on a magnetic medium and, more particularly, to such a pick-up device which is capable of reading out information from the medium regardless of whether that medium is stationary or is moving. This invention is further directed to a method and an apparatus for using that pick-up device for enabling magnetically recorded information to be optically read out.
Typical magnetic reproducing, or playback, apparatus which is used to reproduce information which has been recorded on a magnetic medium, such as magnetic tape, includes a magnetic transducer, or head, which is used to produce electrical signals corresponding to the magnetically recorded patterns which represent the information. A conventional type of playback head is a ring-type head which, normally, is not responsive to a magnetic field. This means that, in order for this head to reproduce the information which is recorded on the magnetic medium, there must be movement between the head and medium. Furthermore, in order to obtain a desirably high signal-to-noise (S/N) ratio, the width of the track in which the information is recorded on the magnetic medium must be relatively wide, that is, on the order of more than about 30 microns, (.mu.m). Because of this wide track width, the recording density is significantly limited. Hence, when the ring-type magnetic head is used to reproduce video signals from a magnetic medium, wide track widths must be recorded, thus resulting in a relatively inefficient use of that medium for video recording.
A magnetic head which is sensitive to the magnetic field or magnetic flux generated by information recorded on a magnetic medium has been proposed. However, such a head requires AC biasing for its operation. This means that a biasing coil and an oscillator must be provided. This often presents difficulties in assembling and constructing magnetic playback apparatus. Furthermore, the maximum frequency which can be reproduced by such a head is limited by the frequency of the biasing signal supplied thereto.
Another type of magnetic head which is responsive to the magnetic flux generated by information recorded on the magnetic medium includes a Hall-effect element. While this type of head advantageously can read out signals while the magnetic medium is stationary, the response of this head is temperature-sensitive because the Hall-effect elements are constructed of semiconductor material.
Yet another type of magnetic playback head is comprised of a combination of a magnetoresistive element and a ring-type head. The resistance of the magnetoresistive element varies as a function of the magnetic field generated by the signals recorded on the magnetic medium. However, since the ring-type head is used, the aforenoted problem of relatively low recording density caused by a track width on the order of at least 30 .mu.m is present.
It is known that a magnetic field will affect a light beam. The Kerr effect produces a rotation of the polarization of polarized light which is reflected from the surface of a magnetized substance. The Faraday effect produces a rotation of the polarization of polarized light which is transmitted through a magnetic substance. It may be thought, therefore, that if a polarized light beam is transmitted to a magnetic medium upon which information is recorded, then the polarization of the light beam which is reflected from that medium can be detected so as to sense rotations therein and thereby decode the recorded information. However, a typical magnetic medium, such as conventional magnetic tape, has an uneven reflecting surface. Consequently, the reflected light beam cannot be detected accurately and, moreover, generally is accompanied by substantial noise signals. Hence, the Kerr effect is impractical to read out magnetically recorded information directly.
One embodiment for utilizing the Faraday effect to read out information from magnetic tape is described in Japanese Patent Publication No. 5483/63. In this apparatus, signal information is recorded by conforming the magnetic domains in a film of hard magnetic semiconductor crystal to desired information patterns. However, this requires a specific type of magnetic medium for recording and, additionally, a special type of read-out system must be used. The information is recorded as a stripe pattern of magnetic domains in the film, and these patterns are detected by transmitting polarized red light through the film and onto a slit. An analyzer positioned behind the slit senses the variations of the polarization due to the Faraday effect in accordance with the magnetic domain stripe pattern. The film generally is on the order of less than 1 .mu.m in thickness; and this significantly limits the applications and usefulness of this type of apparatus. The read-out apparatus described in this publication cannot be used to reproduce the information which is recorded on conventional magnetic media now in general use.
Although the Kerr effect is not practical for use in the direct optical read-out of magnetically recorded information from the magnetic medium per se, a pick-up device may be used in order to adopt the Kerr effect for information read-out. A layer of soft magnetic material, such as permalloy, having its easy axis parallel to the plane of the layer can be mounted on a magnetic medium. The information which is recorded on the magnetic medium is transferred onto the permalloy layer so that when a polarized light beam is focused onto the permalloy layer and is reflected therefrom, the angle of rotation of the polarization can be detected and used to decode the magnetically recorded information. However, the use of permalloy provides relatively poor contrast in detecting the recorded information. Furthermore, the angle by which the polarized light is rotated is very small, such as on the order of about 20 minutes, with the result that the read-out signal has an inferior signal-to-noise ratio.
An improvement in the contrast and signal-to-noise ratio of the read-out signal is obtained if a layer of semi-hard magnetic material having its easy axis normal to the plane of the layer is used as the magnetic transfer medium. One example of such material is an amorphous GdFe film. When this film is brought into contact with the magnetic medium, a magnetic domain pattern is formed in the film corresponding to the recorded pattern in the magnetic medium. This "printed" pattern in the film then can be read by using the Kerr effect. However, and as described in the article "Amorphous GdFe Film Observable with High Contrast by Transferring Magnetic Record Pattern", published in Nikkei Electronics, Jan. 24, 1977, pages 35-37, this technique requires that the GdFe film be demagnetized, or erased, prior to each use thereof to print the recorded magnetic pattern. Also, when printing the information which is magnetically recorded on the magnetic medium, both the medium and GdFe film must be stationary. Consequently, if this film is to be used for reading out the information recorded on magnetic tape, the tape must be stopped, the film then must be brought into contact therewith, the induced magnetic patterns in the film then must be read, and then the film must be erased while the tape is advanced so that the next area can be printed.
Another type of system which has been proposed for reading out magnetically recorded information is described in U.S. Pat. No. 4,052,747. That system uses a magnetic bubble domain device for generating bubble domains in response to the magnetic patterns which are recorded on the magnetic medium. However, when a bubble domain device is used in this system, a domain transport source must be provided to shift the bubble domains through the device and, also, a bias field is necessary to stabilize the bubble domains and a bubble domain annihilator must be provided to erase the device following each reading operation. This results in a relatively complex and impractical read-out system.